LIGA is a microfabrication technology that enables the fabrication of ultraprecise deep microstructures, from hundreds to thousands of micrometers thick, with lateral dimensions in the micrometer range, and submicron tolerances in a variety of materials. It is a sequence of process steps combining deep X-ray lithography, plating-through-mask and replication. The first step of deep lithography in thick resist using synchrotron radiation will be emphasized. The replication techniques extend the applicability of LIGA by offering a broad spectrum of materials but also by allowing for low-cost volume application. Key technological areas receiving attention are materials base expansion, multi-level processing, formation of complex shapes, and assembly.
In this work, a negative-tone photoresist, SU8, used in UV- based micromachining of high-aspect-ratio MEMS has been tested using proximity X-ray printing. Very thick (a few hundreds of micrometers) SU8 resist layers were processed with standard cleanroom equipment and exposed with 1 - 10 keV X-rays at a beamline of the CAMD synchrotron radiation facility. It showed a large increase in sensitivity in deep X-ray lithography compared to the standard poly(methyl-methacrylate) (PMMA) resist, resulting in increased throughput potential. Resist microstructures with aspect-ratio as high as 50 (height 350 : width 7) and vertical sidewalls, were produced. The benefits of using such X-ray resist in X-ray manufacturing are discussed.
We discuss here the capability of direct manufacture of various high- resolution diffractive optics, in particular regarding micromachining of DOEs in 3D. Preliminary demonstrations were made in 2-D using an automated FIB system operated at 30 KeV with a Gallium liquid metal ion source and equipped with a gas injection system (GIS). Gratings with a 20 nm line width and zone plates with 32 nm outer ring were milled in a reactive atmosphere (iodine) directly through 3.5 (mu) m and 800 nm of gold respectively. Plans for combining FIB and X-ray lithography to make diffractive optical elements (DOEs) for JPL are also mentioned.
We discuss here the capability and details of direct manufacture of various high-resolution diffractive optics using focused ion beam (FIB) micromachining. In preliminary demonstrations illustrating the capability of the technique, zone plates and gratings were milled in gold using a reactive atmosphere in an automated FIB system operating at 30 keV with a Gallium liquid metal ion source equipped with an iodine beam gas injection system (GIS). Gratings with 20 nm line width and zone plates with 32 nm outer ring were milled directly through 3.5 micrometer and 800 nm of gold respectively.
This work relates to a method for increasing the adhesion of polymer resist to electrically conductive substrates which is an important step in the lithographic steps for creating high-aspect-ratio micro structures. Here we are particularly interested in plating Ni-Co into the very tall high-aspect- ratio accurately patterned polymethyl methacrylate (PMMA) micromolds. They were made by deep x-ray lithography for primary or secondary metal structures or metal mold inserts within the framework of the LIGAS process. We investigated the effect of using a Novolak intermediate layer on various substrates to consolidate the adhesion of the relatively weak PMMA-metal interface. Modifying our process by introducing this intermediate Novolak sublayer improved the adhesive properties overall throughout the whole process including the planarization step, leading to a more reliable process with better yield as well an increase in the quality of the Ni-Co parts. The increase of bond resistance to heat and x-rays was evaluated by shear stress measurements.
Synchrotron radiation, its production and some applications are presented with special regards to the main characteristics needed by most users and the various optics on for generating synchrotron light from bending magnets, undulators, and wigglers. In parallel to the development of various generations of increasingly powerful synchrotron light sources, compact and simply designed synchrotrons have been built for maximum x-ray output and uptime for industrial use or as smaller regional facilities. The use of synchrotron radiation for industry-related applications has increased continuously during the past 10 years and the demands in some fields like x-ray protein crystallography are expected to grow dramatically.
New developments for x-ray nanomachining include pattern transfer onto non-planar surfaces coated with electrodeposited resists using synchrotron radiation x-rays through extremely high-resolution mask made by chemically assisted focused ion beam lithography. Standard UV photolithographic processes cannot maintain sub-micron definitions over large variation in feature topography. The ability of x-ray printing to pattern thin or thick layers of photoresist with high resolution on non-planar surfaces of large and complex topographies with limited diffraction and scattering effects and no substrate reflection is known and can be exploited for patterning microsystems with non-planar 3D geometries as well as multisided and multilayered substrates. Thin conformal coatings of electro-deposited positive and negative tone photoresist have been shown to be x-ray sensitive and accommodate sub-micro pattern transfer over surface of extreme topographical variations. Chemically assisted focused ion beam selective anisotropic erosion was used to fabricate x-ray masks directly. Masks with feature sizes less than 20 nm through 7 microns of gold were made on bulk silicon substrates and x-ray mask membranes. The technique is also applicable to other high density materials. Such masks enable the primary and secondary patterning and/or 3D machining of Nano-Electro-Mechanical Systems over large depths or complex relief and the patterning of large surface areas with sub-optically dimensioned features.
Low-cost and simple fabrication processes of masks for deep x-ray lithography are necessary for rapid prototyping of high-aspect-ratio microstructures by LIGA-like processes. Commercially available 4' diameter boron carbide substrates of 370 micrometers thickness were investigated as candidates for x-ray transparent mask blanks. High aspect ratio absorber structures were formed by gold electroplating after a x-ray lithographic step using an intermediate KaptonTM mask. The characteristics for boron carbide masks in terms of x- ray transparency and mask contrast were assessed by simulating exposures into polymethyl methacrylate (PMMA) resist using the wavelength shifter at CAMD and compared to other materials combinations, in particular silicon and graphite-based mask blanks. The boron carbide-based masks proved easy to fabricate and were replicated into 300/500 micrometers PMMA resist at intermediate x-ray energies (5 - 15 keV) on a bending magnet beamline of the CAMD ring operating at 1.5 GeV electron energy. These masks were designed for exposure at high photon energy into very thick resist or stacked exposures and will be tested using the CAMD superconductive wavelength shifter as a source of hard x- rays and associated new high energy x-ray lithography station, when available for use.
Processing technique for fabrication of high-aspect ratio structures in PZT is developed in this work using deep X-ray exposures at the CAMD synchrotron storage ring. Arrays of posts were successfully fabricated in PMMA mold as thick as 2700 micrometers and with aspect ratio as high as 15. This LIGA- like technique allows fabrication of structures of shapes and sizes not possible with standard techniques using bulk PZT material or by the existing thin film techniques for PZT deposition. The process shows promise for various applications including high resolution medical imaging and optical projection.
The use of hard X-ray energies for ultra-deep X-ray lithography requires a thorough re-investigation of all issues associated with the LIGA technology materials issues and processes, in particular for the manufacture of high-energy-X- ray masks. Calculations were performed to compare various mask blanks in particular thick KaptonR and thinned silicon blanks. Absorber pattern formation schemes have been investigated using UV contact printing or X-ray lithography with SU8 photoresist. SU8 photoresist also offers an improved X-ray sensitivity over PMMA resist. Resist patterns over 500 micron deep with aspect ratio over 10 and vertical sidewalls were achieved in SU-8, allowing the use of medium energy range X-rays to obtain high quality patterns of much greater resist thickness.
The quality of surface finish and precise height of LIGA components produced by electroplating has been studied. Planarization is often required to meet specifications for both basic LIGA processing (single level) and extended LIGA processing (multi-level). The HI-MEMS Alliance has successfully developed such a planarization process.
A new synchrotron radiation exposure station dedicated to deep-UV exposures has been installed at the synchrotron light source at the Center for Advanced Microstructures and Devices (CAMD). It complements the activities in synchrotron-based lithography including X-ray lithography, deep X-ray lithography, and under way, ultra-deep X-ray lithography. The UV station branches out of the X-ray lithography beamline. A retractable Si mirror reflects the incoming synchrotron radiation beam by 90 deg through a CaF2 window. Three insertable bandpass filters allow the selection of broad-band transmission spectra around the wavelengths of two excimer lasers at 248 nm (KrF) and 193 nm (ArF), and at a shorter wavelength of 187 nm. The station allows for exposures under vacuum or in an inert gas atmosphere.
Proximity printing using synchrotron x-ray lithography provides high resolution pattern transfer with large depth of field, low diffraction effects and no reflection form the substrate. Electro-plating of photo-resist allows deposition of thin, uniform films over geometrically complex and topographically diverse, electrically conductive surfaces. Two electro-deposited photoresists produced by Shipley, EAGLE 2100 ED negative tone and PEPR 2400 positive tone resist, have been tested with x-rays demonstrating micron pattern transfer over depths-of-field in fractions of millimeters.
In thick photoresist applications, commercially available acrylic sheets are bonded to a substrate as an alternative to the casting and in-situ polymerization of PMMA. The factors affecting the adhesion of a thick acrylic sheet to different substrates have been studied. In case of copper and titanium substrates and bond-strength can be improved by roughening the surface through chemical oxidation which then provides a mechanical interlocking between the resist and substrate surfaces. Annealing of PMMA sheet before gluing and use of adhesion promoter such as organosilane further improves the bond strength at the resist-substrate interface. The resist adhesion to various substrates is evaluated by measuring the debonded length of the acrylic sheet during a mechanical cleaving test.
A low-cost mask fabrication process for deep x-ray lithography is described. The mask consists of Kapton films stretched on a ring with absorber structures formed by optical lithography using NFR015 resist and gold electroplating, similar to the masks from the 'early x-ray lithography age'. Such masks proved easy to fabricate and are presently being evaluated for deep x-ray lithography applications. First experiments indicate that they exhibit sufficient radiation resistance and limited variational dimensional changes during exposure at 1.3-1.5 GeV on the XRLM3 beamline at CAMD.
The X-ray lithography and micromachining facility at CAMD hosts the `print-shop' for the development and prototype exposures in LIGA-like processing techniques for the HI-MEMS Alliance. A simple fixture with alignment, tilt, and rotation modules has been developed. It allows for multiple level exposures with registration. More complex shapes can be achieved by rotating and tilting the mask/wafer assembly with respect to the incident X-ray beam. The alignment system is based on optical registration using an X-ray mask with targets on optically transparent windows. The masks were fabricated at MCNC. The alignment tests and off-axis exposures were performed at CAMD. Overlay accuracy of +/- 5 micrometers has been demonstrated.
We have measured the dimensional variation and sidewall roughness of features on PMMA microcomponents fabricated by deep x-ray lithography in order to assess the effect of dimensional variation on subsequent assembly operations. Dimensional measurements were made using a stylus profilometer with a repeatability in step height of better than 0.01 micrometers . Roughness measurements were made with the same profilometer scanning in a direction perpendicular to the length of the parts. 22 micrometers and 54 micrometers features exhibited dimensional variations described by a Gaussian distribution with standard deviations of 0.202 micrometers and 0.381 micrometers , respectively. This corresponds to a maximum relative variation of between 0.6% and 0.9%. Sidewall roughnesses were found to be in the range of 0.02 micrometers to 0.03 micrometers , an insignificant contribution to the total variation when compared to overall dimensional variation. Several potential sources of this variation are discussed, but no single cause was identified as the source of the significant dimensional variation observed here.
The MicroSystems Engineering Team ((mu) SET) at Louisiana State University, in close collaboration with the Center for Advanced Microstructures and Devices, has successfully completed the lithography and electroplating steps of the LIGA process sequence using cyanoacrylate to bond a PMMA resist layer to a nickel surface. Nickel microstructures 300 micrometers in height have been electroplated. Tests were performed which indicate that the bond between cyanoacrylate and nickel is much stronger than the bond between PMMA and nickel.
Reid Brennen, Michael Hecht, Dean Wiberg, Steven Manion, William Bonivert, Jill Hruby, Marcus Scholz, Timothy Stowe, Thomas Kenny, Keith Jackson, Chantal Khan Malek
We are fabricating sub-collimating X-ray grids that are to be used in an instrument for the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. The HESSI instrument consists of twelve rotating pairs of high aspect ratio, high Z grids, each pair of which is separated by 1.7 meters and backed by a single Ge detector. The pitch for these grid pairs ranges from 34 micrometers to 317 micrometers with the grid slit openings being 60% of the pitch. For maximum grid X-ray absorbing with minimum loss of the solar image, the grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. For our proof-of-concept grids we are implementing a design in which a 34 micrometers pitch, free-standing PMMA grid is fabricated with 20 micrometers wide slits and an 800 micrometers thickness. Stiffeners that run perpendicular to the grid are placed every 500 micrometers . After exposure and developing, metal, ideally gold, is electrodeposited into the free-standing PMMA grid slits. The PMMA is not removed and the metal in the slits acts as the X-ray absorber grid while the PMMA holds the individual metal pieces in place, the PMMA being nearly transparent to the X-rays coming from the sun. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met.
We are currently investigating the fabrication of high precision, miniaturized, electrostatic deflectors for use in electron or ion beam micro-columns. These columns can be used in a broad array of applications including microscopy, spectroscopy and lithography. Typically, micro-columns consist of a field emitter tip, a set of micromachined miniaturized lenses and one or more electrostatic deflectors. Miniaturization of the column allows the use of simple electrostatic lenses to achieve very high performance in a package that is just a few millimeters in length. Presently, all reported microcolumns have included miniaturized but conventionally-machined octupole deflector plates. If micromachined plates are used instead, lower deflection voltage is required for deflection, and the system becomes more amenable to very high speed operation. In addition, some reduction in scan field distortion is expected. These improvements results directly from the higher degree of miniaturization, tighter dimensional control, better placement accuracy, and smoother facets offered by micromachining. Given the dimensions (100 micrometers - 1000 micrometers thick) and tolerances (1 - 10 micrometers ) required, LIGA is well suited to fabricate such miniature deflectors. This paper will describe the fabrication of the deflectors using LIGA. The Center for X-ray Optics has built an endstation at Lawrence Berkeley National Laboratory's Advanced Light Source suitable for LIGA X-ray exposures.
Two types of linear multilayer gratings have been investigated with special attention on the influence of the fabrication method on performance. Rh/C and Mo/Si systems have been realized for use above the carbon K-edge at 43.6 A and the silicon L3 edge at 125 A, respectively. We analyzed in detail the performance of the multilayer coatings in relation with their structural behavior. Mo/Si amplitude gratings were manufactured by suppressing the soft X-ray reflectivity of the multilayer mirror in selective areas with gold coating and lift off process. This method provides a well-defined 3 micron period Au grating. The soft X-ray reflectivity of the multilayer alone reached a maximum of 45 percent at normal incidence, whereas it was measured around 12.5 percent after Au grating deposition. Up to 13 grating orders were detected in the grating scan and detector scan showing the very good quality of these structures. A more unusual method was applied to manufacture Rh/C multilayer gratings. A carbon grating was first patterned on a silicon substrate and the multilayer was deposited at the end of the process. Measured reflectivity around 60 A in conventional 0 - 20 scan shows a reduction of the performances by a factor three. This is probably due to the surface roughness of the carbon grating prior to the multilayer deposition.
We report on the fabrication of a laminar multilayer amplitude grating, characterization in the soft x-ray region, and modeling of its properties. Holographic lithography was used to produce a 0.24-μm spatial period grating on a triode sputtered Mo/C multilayer mirror. The pattern was transferred into the multilayer mirror by reactive ion etching in an SF6 plasma after an intermediate lift-off step. The position and relative efficiency of the different orders of a grating etched down to the silicon substrate were measured at the Cu Lαβ line (1.33 nm). The results were interpreted in the framework of a scalar kinematic diffraction theory.
The fabrication of linear, circular, and elliptical reflective zone plate lenses by a multistep process using microfabrication technologies is reported. The diffractive elements were generated by electron-beam lithography on a Mo/C multilayer mirror obtained by means of the triode sputtering technique. The patterns were transferred anisotropically into the multilayer mirror by reactive ion etching in a fluorinated plasma. An intermediate metallic mask made by the lift-off process was used for the transfer process. The groove depth could be monitored by following the reflectivity ofthe structure with a helium-neon laser during etching. Linear, circular (with 0.4-μm outer zones), and elliptical Fresnel zone plates (with 0.8-μ.m outer zones) were produced. Test patterns with a 50-nm top and 90-nm bottom resolution were produced. The groove profile and dimensional control were also investigated.
We report on the fabrication of linear and circular reflective Bragg-Fresnel zone
plates made by a multistep process using microfabrication technologies. The zone plate
patterns were generated by electron beam lithography on a Mo/C triode sputtered
multilayer interference mirror. The minimum zone size was 0.3 p.m wide. The pattern
was transferred anisotropically into the multilayer by reactive ion etching in a fluorinated
plasma. An intermediate metallic mask made by lift-off was used for the transfer
process. The groove depth was monitored by following the reflectivity of the structure
with a helium-neon laser during the etching process. The groove profile and dimensional
control were considered.
We report on the fabrication of a multilayer linear grating, characterization in the
soft x-ray region and modeling of its perfonnance. Holographic lithography was used to
produce a 0.24 p.m spatial period grating on a triode sputtered Mo/C multilayer mirror.
The pattern was transferred into the multilayer mirror by reactive ion etching in an SF6
plasma after an intermediate lift-off step. The position and relative efficiency of the
different orders of a grating etched down to the silicon substrate were measured at the Cu
Lcz line (1.33 nm). The results were compared to the values calculated within the
framework of a scalar kinematic diffraction theory of relief gratings.
We report on the conception, fabrication, and characterization of a multilayer beamsplitter for use at oblique incidence in the soft x-ray range. Thin film deposition, conventional patterning, and anisotropic etch techniques are used to produce the self-supporting silicon carbide carrier film. Large-area Mo/C multilayer beamsplitters (1 cm2) were fabricated. Experimental results on the reflection and transmission at 1.33 nm are presented and compared with theoretical calculations. The role of the supporting film and the flatness of the structure are addressed. Our process is compared with existing approaches from the literature.
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